Instant?
Pull no punches?
No political correctness?
(Please don't hate me...)
FAT / "thick" / "extra" / etc.
(I am assuming your question includes 'physical' attractiveness.)
She may be the sweetest kindest most wonderful human being "on the inside" (which is certainly very attractive in terms of 'personality traits') - but if she is "too large on the outside" - she is "INSTANTLY unattractive" (using your words verbatim).
Just answering the question honestly.
I do not want to discourage anyone or hurt their feelings.
It does not affect the worth of the person.
However, quite honestly, it [can and] does affect ['physical'] "attractiveness" in the eye of the beholder.
For me, it is a matter of personal preference.
I do not speak for all men.